156 NATURAL HISTORY BULLETIN 



C. Superomarginal plates have only one row of spines. 



D. Dorsal marginal spines small, placed on the outer curva- 

 ture of the plates, most developed distally, seldom reaching the 

 second basal plate. Superomarginal plates large and thick, well 

 rounded above, closely granulated. Fasciolated grooves wide. 

 Adambulacral spines mostly flattened; the two in the second 

 series much at and mostly spatulate or trimcate, aboral and much 

 the larger; those of furrow-series compressed, elongated. 



A, articulatus (Say). 

 DD. Dorsal marginal spines largest on the first two basal 

 plates, which are larger and higher than the next plates. 



A. duplicatm Gray (Young). 

 CC. Superomarginal plates have two rows of spines prox- 

 imally. 



E. Inferomarginal spines mostly two or three to a plate. 

 Superomarginal plates not unusually numerous, about 20-30, 

 large, advancing considerably on the disk and rays; basal ones 

 broad and swollen above surface with close, short granules. 

 Paxallar area not very wide, about twice width of marginal 

 plates. Inferomarginals project laterally notably beyond upper 

 plates and have two marginal spines. Rays of moderate length. 

 Dorsal paxillae crowded, not forming regular transverse rows. 



P. Inner adambulacral spines large and stout, compressed; 

 two in median row very unequal ; aboral and large and stout, flat, 

 trimcate. Dorsal paxillae coronate, with short clanate or capitate 

 spinules; central ones similar. A. dupUcatus Gray. 



F. Paxillar area bears many spinopaxillae. 



Var. variabilis (Ltk.) 

 FF. Inner adambulacral spines three, all slender, divergent ; 

 two in second series unequal ; aboral one obtuse, large and stout, 

 not very flat nor spatulate. Dorsal paxillae stellate, with the 

 spinules slender and acute ; central one erect. 



A. antiUensis Liitk. 



EE. Inferomarginal spines mostly one on each plate. Su- 

 peromarginal plates unusually numerous, (48-50), proximal ones 

 high, narrow and angular above, proximal ones not notably en- 

 larged ; surface with spinules or elongated spaced granules, dis- 

 tinctly larger and longer around bases of spines; fasciolated 

 grooves deep and narrow; spines of proximal plates elongated, 



